The invention relates to time division multiplex (TDM) telecommunication switching systems. More particularly, it relates to telephone switching systems having pulse code modulation as the signal transmission format and having a directed TDM ring circuit running from subscriber to subscriber.
TDM telephone switching systems of this type are known in the art (cf. U.S. Pat. No. 3,586,782). In such a switching system, connections may be established in such a manner that a plurality of time slots are defined by a central timing station provided in a ring-like circuit. Each subscriber station is synchronized to the frame of the time slots as a result of a synchronizing signal sent by the timing station in a synchronizing time slot. For the call setup the calling subscriber stations uses a time slot identified by the timing station as free and sends in that time slot the signal of the called subscriber station. All subscriber stations monitor all the time slots as to the reception of their signal and, where necessary, are switched to the connection state in the time slot in which they receive (or have received) their signals.
In telecommunication switching systems, more particularly telephone switching systems, there is frequently the demand that, in addition to connections between two subscribers, calls known as conference or cut-in calls also be set up between more than two subscribers. Conference calls can be set up in a simple way in switching systems in which the speech information is transmitted by means of analog signals, such that the subscriber lines of the subscribers are interconnected directly or via buffer storages. This results in the blending of the speech signals of all the conferees (cf. U.S. Pat. Nos. 3,293,369 and 3,319,005).
In switching systems operating according to the pulse code modulation principle, conference calls cannot readily be set up by blending the various speech signals, in view of the generally non-linear modulation characteristic. To conduct conference calls in a PCM telephone switching system, it is known (CCITT documents WP 33/XV No. 6-E of August 1965, page 2, item (3) and SP 33/XV No. 9-E of December 1965, page 10, second paragraph) to convert the PCM signals obtained from the speech signals of the conferees through non-linear coding, that is, in accordance with a non-linear modulation characteristic, to a linear code. Stated another way, these signals are "linearized in digital form", to add in binary notation the "linear" PCM signals and thereafter to reconvert the composite signals obtained to the non-linear code.
A method equivalent to the aforementioned known method is also found in the subject matter of U.S. Pat. No. 3,575,591. It is also known (U.S. Pat. No. 3,575,591) to store the "non-linear" PCM signals temporarily in first registers, convert them to the linear code one after another via a common converter, store the linear PCM signals in second registers, add them thereafter in an accumulator and, finally, convert the composite signals obtained to non-linear PCM signals via the converter. U.S. Pat. No. 3,575,591 further recites the routing to each subscriber in a 3-party conference circuit of only the composite signals obtained by adding the PCM signals of the other two subscribers.
This known conference circuit utilizes a concentrator switching system to which the individual subscriber stations are connected. It is not suited for digital telecommunication systems without an exchange and having a ring circuit running from subscriber station to subscriber station.
It is an object of this invention to provide a method of connecting in a conference call at least 3 subscribers to a TDM telecommunication switching system, more particularly a PCM telephone switching system, having a directed TDM ring-like circuit running from subscriber station to subscriber station.